Method of cleaning and/or regenerating wholly or partially de-activated catalysts of stack-gas nitrogen scrubbing

ABSTRACT

A cleaning kit for removing process impurities carried on the surface of a NO.sub.x reduction catalyst which is installed in the path of a flue gas flow exiting from a fossil fuel burning facility including: a reagent supply grill; a source of supply of liquid cleaning reagent; a reagent collection basin; and a recirculating structure. The reagent supply grid is adapted to be selectively positioned above a portion of a catalyst layer. The source of supply of liquid cleaning reagent adapted to be in communication with the supply grid, and the reagent collection basin being adapted to be selectively positioned below the portion of such catalyst layer to catch cleaning reagent therein after such reagent passes through such portion of the catalyst layer, the recirculating structure recirculates at least a portion of such reagent from the collecting basin for recirculating through such supply grid for further cleaning of such portion of the catalyst layer.

The invention relates to a method for scrubbing and/or regenerating ofwholly or partially deactivated catalytic devices for nitrogen removalfrom stack gases, wherein the catalytic devices are treated with ascrubbing, or respectively regeneration fluid.

Such catalytic devices are also called SCR (selective catalyticreduction) catalytic devices. The deactivation of such catalytic deviceshas several different causes, mainly:

Clogging of the honeycomb structure, or respectively the free spaces inthe catalytic device. Because of this, the stack gas does not reach thecatalytic device and the clogged conduit of the catalytic device is notused for the catalytic reaction. In order to use the installed catalyticmaterial as efficiently as possible, attempts are made to decrease theclogging of honeycomb channels or plate channels by cleaning measures,such as steam blowers in the DENOX installation or manual cleaningactions. In spite of this, some of these honeycombs, or respectivelyfree spaces in the catalytic device, become clogged over time. With someinstallations the catalyst modules are removed and placed on anappropriate shaking device. The clogs are loosened by the shakingmovements. In this way the stack gas again gains access to the catalyticmaterial. The increase in activity does not constitute a regeneration,it only provides access to the clogged catalytic material. The surfacelayer being formed during operation remains untouched by this cleaningstep.

Worsening of the gas diffusion at the surface of the wall of thecatalytic device because of the growth of a thin surface layer ofapproximately 1 to 100 μm and clogging of pores. Because of this, thestack gas can only reach the pores of the catalytic material poorly ornot at all. The formation of a thin surface layer worsens the chemicaltransformation of NO_(x) and NH₃ into N₂ and H₂O, because the gasdiffusion into the catalytic material is greatly hampered.

Clogging of the active catalytic centers on the surface of the catalyticdevices by means of the accumulation of the so-called catalytic poisons,for example As, K, Na. The settling of catalytic poisons, such asarsenic, for example, on the active centers of the catalytic devicemakes the reaction at these centers impossible and in this way also aidsin a reduction of the activities of the catalytic material.

Abrasion of catalytic material by solids, such as fly ash, contained inthe stack gas. The catalytic material is reduced because of the loss ofcatalytic material and therefore of the surface available for thereaction. The abrasion of catalytic material is an irreversible processwhich results in a permanent loss of activity. The following actions canalso simultaneously occur in the course of abrasion by fly ash:

-   -   Removal of catalytic material and of an existing surface layer,    -   Retention of components of the fly ash and therefore formation        of a fresh gas diffusion-hindering surface layer.

A method is described in DE 38 16 600 C2 in which the regeneration ofcatalytic devices contaminated by arsenic is described. This method doesnot take into consideration the portion of the deactivation by a gasdiffusion-hindering surface layer. Aqueous solutions of nitric acid,hydrochloric acid, sulfuric acid or acetic acid are employed as thescrubbing suspension in the method according to DE 38 16 600 C2. Thesescrubbing suspensions have the disadvantage that for one they are tooexpensive and also that the disposal of the acids contaminated byarsenic is elaborate.

A method is described in EP 0 136 966 B1, in which initially the dustadhering to the surface is removed with dry steam. The catalytic poisonsare then intended to be dissolved and rinsed out in a second step by wetsteam with a moisture content of ≦=0.4. Drying is performed with drysteam again. In the method in accordance with EP 0 136 966 B1, the thin,gas diffusion-hindering layer is not removed in a first step, insteadclogged conduits are merely opened again. This has already been done ona large-scale basis for a long time in the form of so-called dust orsoot blowers. The second step of this method can have anactivity-increasing effect only with catalytic devices wherein the gasdiffusion-hindering layer does not exist over the entire surface or notat all. Also, the generation of large amounts of dry and wet steam isvery energy-intensive.

A method for the reactivation of catalytic devices is described in DE 3020 698 C2, which removes the deactivating substances by means of adefined pressure and a defined temperature. Various gases, for examplemethane, propane, carbon dioxide or argon can be added in the processfor optimizing the method. This method also does not consider the gasdiffusion-hindering surface layer.

A great disadvantage of most of the mentioned methods is the fact thatthey can only be performed in a separate installation. To this end theremoval of the catalytic devices and therefore an outage of theinstallation is required.

Accordingly, it is the object of the invention to further develop amethod of the type mentioned above in such a way that gas diffusion onthe surface of the catalytic devices is again made possible, whereinadditionally the clogging of the active centers by catalytic poisons isreversed to the greatest extent possible, and which can be performedinside the nitrogen removal installation without the removal of thecatalytic devices.

This object is attained in that the scrubbing, or respectivelyregenerating fluid is fully desalinated water.

The function of the invention is based on the dissolution and removal ofthe surface layer for restoring the gas diffusion and exposing of activecenters for the nitrogen-removing reaction of the surface of thecatalytic device. In this case the composition of the fluid must beselected in such a way that, along with a small consumption ofregenerating suspension, the fastest possible dissolution of the surfacelayer is achieved. In connection with the regeneration of SCR catalyticdevices it has surprisingly been shown to be useful to employ fullydesalinated water, for example demineralized water, for dissolving thesurface layer. The use of demineralized water as the scrubbing fluidprevents the introduction of catalytic poisons with the scrubbing fluid.In comparison with other possible fluids, demineralized water has theadvantage that it is relatively inexpensive and that in most cases itcan be produced at the location of the power plant itself. The cleaningand regeneration of the catalytic devices is performed at ambienttemperatures, so that no energy is required for heating the fluid. Bymeans of this method it is possible to drastically reduce the number ofdeactivated catalytic devices to be disposed. Above all, in largeinstallations for the reduction of nitric oxides, so-called DENOXinstallations, this method is suitable for regenerating the used anddeactivated catalytic devices, i.e. to again increase the reducedcatalytic activities, without having to remove them.

An advantageous further development of this method provides, that thecatalytic devices are first mechanically cleaned by vacuuming or blowingthe deposits out, which is then followed by a scrubbing cycle, whichremoves the surface layer by means of a regenerating suspension anddissolves the clogs of the active centers to a great extent. It has beenshown to be advantageous for the consumption of regenerating suspensionif only a small portion of the regenerating suspension is continuouslyremoved and regenerated, i.e. the larger part can be employed in arecirculating operation.

An additional opportunity for reducing the scrubbing water is the use ofa suitable abrasive which only removes the surface layer. This methodcan also be practiced inside the nitrogen removal installation. Theabrasive (for example small glass spheres), together with the parts ofthe gas diffusion-hindering surface layer, can then be disposed oftogether with the fly ash from the electronic filter.

Further advantageous developments of the invention are defined in thedependent claims.

An exemplary embodiment for the use of a suitable regeneration devicewill be described in greater detail in what follows, making reference tothe attached drawings. Represented are in:

FIG. 1, a schematic structure of a catalytic device strip with surfacelayers,

FIG. 2, the enlargement of a portion of FIG. 1,

FIG. 3, a method flow graph for the cleaning of catalytic devices insidea DENOX installation,

FIG. 4, a schematic view of the cleaning of the catalytic device bymeans of an abrasive.

FIGS. 1 and 2 show an enlarged sectional view through a catalytic devicestrip 60 of a catalytic device 6. A catalytic device strip 60 of ahoneycomb catalytic device with pores 61 is represented. A surface layer62 of a thickness of approximately 1 to 100 μm grows with increasinglength of operation which, with increasing thickness, more and morehinders the diffusion of the stack gas to be cleaned into the catalyticmaterial, in particular the pores 61.

An exemplary embodiment of the present invention becomes clear by meansof the flow graph of the method represented in FIG. 3.

A container 11 is filled with desalted water, for example demineralizedwater, from the complete desalination installation of a power plant, viaa line 1. Additives can be supplied to the scrubbing fluid via lines 2and 3, for example hydrochloric acid for lowering the pH value, orregenerating substances, such as vanadium, molybdenum or tungsten, forexample. The pump 4 conveys the regenerating suspension through the line5 into the DENOX installation 17, where the catalytic devices 6 arescrubbed. The scrubbing fluid with the materials contained in thesurface layer and the catalytic poisons are conducted via a suitablecatching device, for example a funnel, and a pump 7 to a separatingdevice 8. There, the materials contained are separated in a suitablemanner from the scrubbing fluid. A hydrocyclone, for example, issuitable for this. However, filters or the like are also conceivable.The underflow from the separating device 8, which is heavily loaded withsolids, is conveyed via the pump 16 to a settling tank 9. The solidcomponents are further concentrated in this settling tank 9, are drawnoff in a partial flow via a line 10, and conveyed to a suitable wastewater treatment, not represented here. The overflow of the settling tank9 and the upper flow of the separating device 8 are conveyed to thecontainer 11 via the lines 12 and 13 and pumps 14 and 15.

This structure can be expanded by suitable precipitation stages, inwhich dissolved noxious matter, such as the catalytic poison arsenic,for example, is precipitated, so that it can be separated by means ofthe separating device 8 and removed from the scrubbing fluid. Thescrubbing, or respectively regenerating fluid is conveyed in circulationin this way, from which only a defined volume of fluid with theconcentrated noxious matter, is removed per circuit. This volume isreplenished through the lines 1, 2 and 3.

A further possibility for execution is closing the honeycombs of thecatalytic device, or respectively of the reactor, below the catalyticdevice 6. The catalytic devices are thereafter filled with thescrubbing, or respectively regenerating fluid. During this bath in theregenerating fluid, first the gas diffusion-hindering surface layer isloosened. The catalytic poisons inside the pores of the catalytic deviceare then loosened from the active centers on the surface of thecatalytic device and are transferred into the regenerating fluid.Because of the concentration drop between the regenerating fluid insidethe pores of the catalytic device and the regenerating fluid in thehoneycomb channels, the dissolved catalytic poisons move to thehoneycomb channels. After a defined period of time the regeneratingfluid with the components of the gas diffusion-hindering surface layerand the catalytic poisons is drained. The catalytic devices arethereafter dried by means of stack gas or hot air. The advantage of thisembodiment lies in the low consumption of regenerating fluid.

Complementing the mentioned exemplary embodiments it is also possible toconnect the regeneration of catalytic devices directly with drying. Inlarge nitrogen-removing installations it can occur that some tons ofregenerating fluid still remain in the catalytic devices 6. Thestructural steel for receiving the catalytic modules must be designedfor this additional weight. This is not the case in some installations.It is then necessary to dry a partial section immediately after theregeneration of this section. In the course of this, the catalyticdevices 6 are first regenerated as described. Following regeneration,the regenerated section is dried by means of hot air or hot gas. Bymeans of this the regenerating suspension remaining in the catalyticdevices 6 is evaporated and removed.

FIG. 4 shows in a schematic representation a complementing option forremoving the surface layer 62 from the catalytic devices 6. An abrasive63, for example sand or glass, is used for mechanically removing thesurface layer 62. The abrasive 63 is blasted through a tube 64 or thelike on the surface 65 of the catalytic device 6. The abrasive material66, which has been contaminated with portions of the surface layer, isblown out of the catalytic device 6, or rinsed out during cleaning withthe scrubbing fluid, for example.

EXAMPLE

The invention was tested on used and deactivated catalytic devices. Tothis end, a deactivated catalytic element of a total length of 840 mmand edges of the length of 150×150 mm was removed from a DENOXinstallation and treated in accordance with the regenerating method.Prior to regeneration with demineralized water, the catalytic elementwas examined in a test stand. The catalytic element was thereafterrinsed for 5 minutes with demineralized water and subsequently driedwith hot air. A subsequent examination showed that the NOX precipitationrate was increased by approximately 5% to 6% over the entire mol ratiorange of NH₂/NOX of 0.8 to 1.2, as shown in the following table. Molratio Nh₂/NOX 0.8 0.9 1.0 1.1 1.2 NOX precipitation rate 64.8 70.6 73.775.2 76.4 before regeneration NOX precipitation rate 70.4 75.8 78.9 80.681.8 after regeneration

1-13. (canceled)
 14. A method for treating wholly or partiallydeactivated catalytic devices used for nitrogen removal of stack gases,comprising the steps of: treating the surface of the catalytic deviceswith a regenerating fluid to remove contaminants from the surface of thecatalytic devices without removing the catalytic devices from theirnormal operating position; collecting the regenerating fluid and thecontaminants; separating at least a portion of the contaminants from theregenerating fluid; recirculating the regenerating fluid for reuse inthe treatment until the catalytic devices have been regenerated; and,drying the catalytic devices.
 15. The method of claim 14 furthercomprising the step of treating the deactivated catalyst devices with anabrasive prior to treating the surface with a regenerating fluid. 16.The method of claim 14 wherein the regenerating fluid is desalinatedwater at ambient temperature.
 17. A catalyst regeneration system forregenerating deactivated catalytic devices used for nitrogen removal ofstack gasses without moving the catalytic devices from its normaloperating position, comprising: a container for storing a regenerationfluid; a fluid pump for conveying the regeneration fluid to thedeactivated catalytic devices; a treatment system for scrubbing thedeactivated catalytic devices with the regeneration fluid to removecontaminates from the deactivated catalytic devices; a catching systemfor collecting the runoff of the regeneration fluid and thecontaminants; a waste pump for conveying the regeneration fluid and thecontaminates to a separating system; a separating system for separatingthe regeneration fluid from at least a portion of the contaminates; acontaminate transfer system for conveying the contaminates to atreatment area; and, a recycling pump for conveying the regenerationfluid in the separating system to the container for reuse in thecatalyst regeneration system.
 18. The catalyst regeneration system ofclaim 17 further comprising a drying system for drying the catalyticdevices once the catalytic devices have been regenerated.
 19. Thecatalyst regeneration system of claim 18 further comprising an additivesystem for combining active catalytic substances with the regenerationfluid.
 20. The catalyst regeneration system of claim 17 wherein theregeneration fluid is desalinated water.
 21. A cleaning system to removecontaminants carried on a surface of a wholly or partially deactivatedNOX reduction catalyst which is installed in a flue gas flow path of afossil fuel burning facility, comprising: a movable cleaning reagentsupply grid adapted to be selectively positioned above a catalyst layer;a supply source of liquid cleaning reagent adapted for communicationwith the supply grid, from which a portion of the liquid cleaningreagent is directed through the supply grid so that essentially all ofthe catalyst layer is contacted by cleaning reagent, the liquid cleaningreagent removing at least a portion of the contaminants from thecatalyst layer; a cleaning reagent collection basin adapted to beselectively positioned below the catalyst layer to catch cleaningreagent after the reagent passes through the catalyst layer; and aseparating device for removing at least a portion of accumulated solidsfrom the at least a portion of such cleaning reagent prior torecirculation through the supply grid.
 22. The cleaning system of claim21 further comprising an additive device for supplying additives to theliquid cleaning reagent.
 23. The cleaning system of claim 22 furthercomprising a dryer.
 24. The cleaning system of claim 23, wherein theseparating device is a hydrocyclone.